How do you perform flight test system identification to extract stability derivatives?

System identification extracts aerodynamic derivatives from flight data: Flight maneuvers designed for observability (doublets, frequency sweeps, multistep inputs), Data acquisition at high rate (50-100 Hz) with synchronized sensors, Preprocessing (filtering, bias removal, kinematic consistency checks), Identification methods - Equation error (least squares fit to equations of motion), Output error (iteratively match predicted response to measured), and Filter error (Kalman filter based, handles noise). Cramer-Rao bounds quantify parameter uncertainty. Validation through prediction of independent maneuvers. Challenges include: Correlated regressors, Limited maneuver amplitude, Sensor errors and delays, and Model structure selection. Results update simulator aerodynamic databases and validate wind tunnel/CFD predictions.